In a large skillet, melt butter on medium high heat. Sear chicken on all sides. No need to fully cook the chicken, just sear to get some color. Transfer chicken to clean bowl.
Add celery, bell peppers and onion to skillet. Cook until onions begin to soften. Add cayenne pepper and Cajun seasoning. Toss to combine. Remove from heat.
In the slow cooker, add diced tomatoes (along with juice) and chicken broth. Add seared chicken and sliced sausage. Follow with celery-onion mixture.
Cover slow cooker and cook for 4-6 hours on low setting.
*If using uncooked rice, add rice to slow cooker. Cover and cook for 30 minutes on high setting.
Once rice is cooked, add frozen shrimp. Cover slow cooker and let shrimp warm up for 8-10 minutes.
*If using cooked rice, add rice after shrimp has warmed up.
Serve warm and garnish with parsley and additional Cajun seasoning as needed.